Simultaneous recordings from pericruciate pyramidal tract and non-pyramidal tract neurons; response to stimulation of inhibitory pathways

Abstract

The excitability of pairs of simultaneously recorded pericruciate pyramidal tract (PT) and non-pyramidal tract (non-PT) neurons was examined in pentobarbital anesthetized cats. The results are summarized as follows. (1) The spontaneous activity of PT neurons was in many instances random, and contrasted with the preponderance of very short or very long interspike intervals which characterized the spike discharge patterns of most non-PT cells. Some non-PT neurons discharged in bursts of 3–10 spikes at frequencies up to 600 cycles/sec. (2) All PT neurons were subject to recurrent inhibition from a pyramidal tract volley. A similar response was obtained from only 40% of non-PT cells, and usually required higher intensities of stimulation. Twenty per cent of non-PT cells were uninfluenced by the stimulus, while excitation-inhibition sequences characterized the responses of the remainder. (3) Inhibition, or excitation-inhibition sequences characterized the pattern of response of both PT and non-PT cells to stimulation of the cortical surface. (4) Transcallosal volleys evoked an inhibitory response, often preceded by a short period of excitation, from all PT neurons tested. Similar responses, although often with a more prolonged initial excitation and a prominent postinhibitory enhanced excitability, were observed from non-PT neurons. (5) Activation of ‘specific’ thalamic nuclei evoked inhibition or excitation-inhibition sequences from both PT and non-PT neurons. A prominent initial excitation c consisting of bursts of several spikes at high frequency was observed from more superficial non-PT cells. This response was followed by a silent period which terminated in sequences of postinhibitory enhanced excitability. (6) Although the duration of the inhibitory pause recorded from both PT and non-PT cells was in general greater than 150 msec, no initial excitation of a similar time course was found among the non-PT neurons examined. Moreover, any initial excitation was usually followed by a prolonged inhibitory pause in excess of 100 msec. (7) The different activity patterns of PT and non-PT neurons may be related to some of the known morphological differences between cortical pyramidal and stellate neurons.